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UNIVERSITY  OF  OKLAHOMA  BULLETIN 
NEW  SERIES  NO.  110.  UNIVERSITY  STUDIES  SERIES  NO.  7 

UNIVERSITY  OF  OKLAHOMA 
BULLETIN 


UNIVERSITY  STUDIES 

T! LIB:  Ai;  i  u«- 

|;  AUG  2  8  1931 

UNIVERSITY  frF  ILL- 
STRUCTURE  AND  FUNCTION  IN  THE  DE- 
VELOPMENT OF  SOME  OF  THE  SPECIAL 
SENSES  IN  MAMMALS. 


H.  H.  LANE,  Ph.  D., 
Professor  of  Zoology  in  the  University  of  Oklahoma 


Norman,  Oklahoma 
July  15,  1916 


University  of  Oklahoma  Bulletin,  published  by  the  university,  is 
issued  semi-monthly.  Entered  at  the  postoffice  at  Norman,  as  second 
class  matter,  under  act  of  congress  of  August  24,  1912. 


STRUCTURE  AND  FUNCTION  IN  THE  DE- 
VELOPMENT OF  SOME  OF  THE  SPECIAL 
SENSES  IN  MAMMALS. 


H.  H.  LANE,  Ph.  D., 
Professor  of  Zoology  in  the  University  of  Oklahoma 


Digitized  by  the  Internet  Archive 
in  2013 


http://archive.org/details/structurefunctioOOIane 


STRUCTURE  AND  FUNCTION  IN  THE  DEVELOPMENT 
OF  SOME  OF  THE  SPECIAL  SENSES  IN  MAMMALS. 

H.  H.  Lane. 


PREFACE. 

The  following  paper  was  read  before  the  Sigma  Xi  Club  of 
the  University  of  Oklahoma  on  the  evening  of  December  13, 
1915.  It  is  to  be  regarded  as  a  resume  or  preliminary  report  on 
a  considerable  amount  of  work  on  the 'correlation  between  struc- 
ture and  function  in  the  development  of  the  special  senses  in 
mammals  done  partly  at  the  University  of  Oklahoma  and  partly 
at  Princeton  University.  Acknowledgment  is  made  of  much  en- 
couragement and  assistance  on  the  part  of  Professor  E.  G. 
Conklin,  Professor  C.  F.  W.  McClure,  and  Doctor  Stewart  Pa- 
ton,  all  of  Princeton  University,  Professor  L.  W.  Cole  of  the 
University  of  Colorado,  and  to  the  National  Academy  of 
Sciences  for  a  grant  of  $500.00  for  the  purchase  of  equipment 
necessary  to  continue  the  work  and  broaden  its  scope  in  a  com- 
parative way.  The  occasion  and  manner  of  its  presentation  will 
explain  the  form  in  which  it  is  here  presented.  The  full  ac- 
count of  the  whole  investigation  will  be  published  elsewhere. 


Possibly  it  may  seem  to  some  of  you  that  your  speaker  owes 
you  somewhat  of  an  apology  for  asking  your  time  and  atten- 
tion to  this  subject  of  structure  and  function  in  the  develop- 
ment of  the  special  senses.  Watson,  of  Johns  Hopkins  Univer- 
sity, in  a  recent  book  on  "Behavior"  (p.  49)  asserts  that:  "Stu- 
dies upon  the  structure  of  sense  organs  and  upon  the  nervous 
system  generally  seem  to  be  somewhat  out  of  fashion  at  pres- 
ent," though  he  is  kind  enough  to  add  (p.  50)  that:  "This  condi- 
tion of  affairs  is  probably  purely  temporary,"  and  outlines  a 
"group  of  problems"  (p.  31)  in  this  field  as  follows:  "The  vast 
majority  of  the  problems  in  both  human  and  animal  behavior- 
may  be  grouped  under  one  or  another  of  three  divisions: 
I.  Sense  organ  functions.  II.  Instinctive  functions.  III.  Habit 
formation.  In  addition  to  these  large  divisions  in  which  the 
subjects  for  research  lie,  there  remains  the  work  of  IV.  Correla- 
tion: first,  among  behavior  data — giving  both  an  ontogeny  and 
a  phylogeny  of  behavior;  second,  of  behavior  with  structure: 


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and  finally  the  correlation  of  behavior  and  structure  with  physi- 
co-chemical processes." 

An  examination  of  Watson's  volume  as  well  as  his  previous 
work,  not  to  mention  that  of  other  investigators  in  his  field,  re- 
veals the  fact  that  he  refers  only  to  the  behavior  of  animals 
after  birth.  Some  time  before  the  statements  quoted  were 
written,  or  at  least  had  appeared  in  print,  I  had  undertaken  a 
study  of  animal  behavior  from  still  another  point  of  view,  name- 
ly, the  prenatal  behavior  of  mammals  and  its  correlation  in 
ontogeny  with  the  development  of  the  structures  concerned. 
Aside  from  some  unpublished  work  of  Paton  on  the  guinea-pig. 
no  work  of  this  kind  on  mammals  is  known  to  me.  Paton  had 
in  1907  published  in  a  Naples  report  an  account  of  some  related 
investigations  on  lower  vertebrates  and  later  two  or  three  brie^ 
papers  on  the  chick.  Coghill,  of  Kansas  University,  and  Her- 
rick,  of  the  University  of  Chicago,  have  obtained  some  very  in- 
teresting and  important  results  from  a  study  of  the  swimming 
reflex  in  salamander  larvae.  A  French  investigator,  Wintrebert, 
had  previously,  in  1904  and  1905,  published  some  brief  notes  on 
some  investigations  which  he  had  made  on  the  axolotl  and  fro^ 
tadpole.  With  these  exceptions,  which,  as  you  may  readily  see, 
are  not  exactly  in  line  with  studies  about  to  be  reported  to  you, 
the  field  of  what,  for  want  of  a  more  exact  term,  may  be  called 
prenatal  behavior  has  been  practically  untouched.  A  full  ac- 
count of  the  experiments  and  observations  made  to  date  will  be 
published  elsewhere,  and  would  in  their  entirety  be  altogether 
too  much  to  attempt  to  give  in  an  hour's  address,  hence  only  a 
resume  is  contemplated  here.  Moreover,  the  problem  is  com- 
pleted in  only  its  broadest  outlines  and  there  yet  remains  much 
to  be  done  to  fill  in  the  details.  The  program  set  for  the  future 
will  require  years  of  labor  to  bring  to  a  successful  conclusion, 
and  will  probably  demand  the  attention  of  many  additional  in- 
vestigators before  its  riches  shall  have  been  exhausted.  The 
greater  part  of  the  investigation  carried  on  for  the  past  two 
years  or  so  has  been  limited  purposely  to  a  study  of  one  form, 
the  albino  variety  of  the  Norway  rat  (Mus  norvegicus  albinus) ; 
but  thanks  to  a  generous  grant  from  the  National  Academy  of 
Sciences  it  is  now  possible  to  extend  the  range  of  the  investiga- 
tion over  a  number  of  species,  in  short,  to  make  the  study  a 
comparative  one,  as  should  be  done  with  all  biological  problems 
of  this  general  nature. 

In  general  terms  the  problem  may  be  stated  as  follows:  by 


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means  of  suitable  physiological  experiments  to  determine  just 
when  the  embryo  first  becomes  possessed  of  the  functions  of 
touch,  equilibrium,  smell,  taste,  hearing,  and  sight;  in  other 
words,  when  it  is  first  capable  of  receiving  sensations  of  those 
sorts,  and  to  determine  furthermore  the  nature  of  the  responses 
V>  is  able  to  make  at  different  stages  in  its  ontogenetic  develop- 
ment and  to  correlate  its  functional  development  with  the  struc- 
tural development  of  the  corresponding  organs,  namely,  the 
central  and  peripheral  nervous  system,  including  organs  of 
special  sense. 

Regarding  the  correlative  relationship  of  structure  and  func- 
tion, two  views  have  chiefly  demanded  consideration  at  the 
hands  of  biologists.  The  first  of  these  regards  structure  as  the 
cause,  or  at  least  as  the  forerunner  of  function;  while  the  sec- 
ond reverses  the  situation  and  regards  function  as  the  cause  of 
structure.  Some,  like  Conklin,  have  sought  to  develop  an  inter- 
mediate position,  and  regard  neither  as  the  cause  of  the  other, 
but  both  as  inherent  properties  of  the  organization  fundamental 
to  living  matter.  The  view  most  commonly  held  regarding  the 
origin  of  organs  of  special  sense,  either  explicitly  or  implied,  is 
that  of  the  Lamarckian  school  of  biology,  namely,  that  they 
arose  in  phylogeny,  at  least,  as  adaptations  to  conditions  of  the 
external  environment.  Light,  for  example,  impinging  upon  the 
surface  of  an  organism  led  first  to  the  deposition  of  pigment  on 
the  exposed  part,  and  this  ultimately  became  connected  with 
the  nervous  system,  and  developed  under  the  influence  of  con- 
"tinued  use  into  a  special  light-perceiving  organ.  The  Darwin- 
ians, of  course,  would  modify  the  last  statement  by  saying  that 
in  cases  where  the  pigment  spot  happened  to  be  connected  with 
the  nervous  system,  natural  selection  preserved  it,  and  develop- 
ed it  into  an  organ  of  special  sense. 

If  this  has  been  the  phylogenetic  history  of  the  eye,  for 
example,  it  would  be  natural  to  expect  that  in  ontogeny  there 
would  be  some  hint  at  this  course  of  development,  though  it  is 
now  a  well  recognized  fact  that  the  recapitulation  of  phylogeny 
in  ontogeny  is  never"  very  exact,  and  often  is  obscured  or  en- 
tirely obliterated.  But  to  find  a  course  of  development  of  even 
one  organ  of  special  sense  exactly  the  reverse  of  that  demand- 
ed by  current  theories  should  make  us  pause,  and  if  a  whole  ser- 
ies of  sense  organs  should  give  concurrent  testimony  adverse 
to  the  generally  accepted  theory,  it  would  appear  that  a  re- 
vision of  that  theory  is  urgently  demanded. 


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With  this  idea  in  mind,  let  us  examine  some  data  from  my 
work  on  the  rat: 

I.  TOUCH: 

The  tactile  sense  is  not  only  the  most  primitive  but  it  is  still 
the  least  differentiated  of  the  special  senses,  so  that  it  presents 
the  logical  point  of  first  attack  upon  this  problem.  Of  the  fif- 
teen stages  of  the  rat  examined,  the  youngest  had  a  length 
(neck-rump)  of  only  7l/2  mm.,  and  gave  no  evidence  of  the 
possession  of  a  sense  of  touch.  Stimulation  with  a  fine  sable 
brush,  or  with  the  prick  of  a  fine-pointed  needle  brought  about 
no  perceptible  response.  The  inductorium  produced  a  change 
in  the  rate  of  the  heart  beat,  but  otherwise  was  no  more  effec- 
tive than  were  the  mechanical  stimuli  just  mentioned. 

When  sectioned  it  was  found  that  a  large  number  of  corre- 
lation or  coordination  fibers  were  already  present  in  the  spinal 
cord  and  brain  stem.  Both  the  afferent  and  efferent  roots  of 
the  spinal  nerves  were  present,  although  the  former  did  not 
reach  the  integument.  In  the  region  of  the  snout  two  branches 
of  the  trigeminal  nerve  could  be  seen  which  however  ended 
within  the  mesenchyme,  i.  e.,  did  not  reach  the  integument  of 
that  region.  The  vibrissae  or  "tactile  whiskers"  so  prominent 
later  in  the  life  of  the  rat  had  not  apparently  begun  to  be 
formed;  at  any  rate  their  anlagen  could  not  be  found. 

The  next  stage  obtained  had  an  average  length  of  16  mm. 
and  will  therefore  be  referred  to  as  the  16  mm.  stage.  By  this 
time  the  tactile  sense  is  present  on  theflanks  and  snout  as  evi- 
denced by  motor  responses  to  the  prick  of  the  needle,  though 
the  more  delicate  stimulus  of  the  sable  brush  was  still  ineffec- 
tive. In  the  former  case  the  response  consisted  of  a  turning 
movement  of  the  head  as  a  whole,  and  of  slight  but  readily  per- 
ceptible movements  of  the  body  when  the  stimulus  was  applied 
to  the  flank.  There  was  at  this  time  no  sign  as  yet  of  the  histo- 
genesis of  muscle  in  the  snout  region,  and  the  turning  move- 
ments of  the  head  were  so  promptly  made  that  there  can  be  no 
question  but  that  the  stimulus  was  transmitted  along  nerve 
paths  and  not  through  the  general  protoplasm.  Furthermore 
the  sections  demonstrate  the  presence  of  about  a  dozen  anlagen 
of  vibrissae  on  each  side  of  the  snout.  These  are  innervated 
by  branches  of  the  maxillary  division  of  the  Vth,  or  trigeminal 
nerve,  which  ends  in  basket-like  reticula  in  the  vibrissal  follicles. 

In  embryos  23  to  28  mm.  long,  stimulation  with  the  sable 
brush  produced  reaction,  as  did  also,  of  course,  the  needle- 


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prick.  Again  it  was  found  that  the  snout  region  is  most  sen- 
sitive, though  by  this  time  stimulation  with  the  needle  about 
the  shoulder,  upper  arm,  hip,  rump,  and  thigh  also  evoked  motor 
responses.  The  sections  show  a  noticeable  increase  in  the 
number  of  the  vibrissal  anlagen  as  well  as  greater  complexity 
in  the  neurofibral  "basket"  in  each  vibrissal  follicle.  The  num- 
ber of  trigeminal  fibers  going  to  a  single  follicle  has  greatly  in- 
creased since  the  16  mm.  stage.  The  general  integument  of 
the  snout  has  not,  however,  yet  received  the  terminations  of 
other  branches  of  the  trigeminus,  although  many  such  are  ex- 
tending toward  it,  for  the  most  part  paralleling  blood  vessels 
in  their  course.  In  the  26  to  28  mm.  embryos  there  are  also 
correlation  or  coordination  paths  running  between  the  medulla 
and  the  mid-brain. 

In  the  3.5  cm.  fetus  and  new-born  rats  the  tactile  sense  was 
still  better  developed  over  practically  the  whole  of  the  body, 
tail,  and  limbs.  The  snout  was,  however,  the  most  sensitive  as 
was  shown  by  the  response  to  stimulation  with  a  single  hair, 
while  elsewhere  it  took  the  mass  pressure  of  the  brush  to  elicit 
a  response.  Pain,  or  at  least  discomfort,  was  shown  in  the  3.5 
cm.  fetus  one  hour  after  removal  from  the  mother's  uterus,  by 
the  emittance  of  squeaks  when  pricked  with  the  needle.  Struc- 
turally speaking,  there  is  in  these  stages  an  increased  number 
of  vibrissae  on  the  snout;  the  anlagen  of  the  body-hairs  are 
very  numerous,  and  the  integument  contains  a  rich  plexus  of 
nerve-fibers  extending,  at  least  in  the  snout  region,  through  the 
stratum  germinativum  into  the  stratum  intermedium  of  the 
epidermis.  There  is  a  notable  increase  in  the  number  of  affer- 
ent fibers  of  the  trigeminus  distributed  to  the  snout  region.  The 
central  connections  are  better  marked  and  more  extensive  than 
in  the  preceding  stages.  The  fibrillar  baskets  in  the  vibrissal 
follicles  are  now  elongated,  felted  cylinders,  from  the  bases  of 
which  the  neurofibers  emerge  in  a  relatively  large  bundle  some 
distance  distad  to  the  base  of  the  follicle  itself. 

Throughout  the  older  stages  examined  there  was  in  general 
no  particular  advance  in  tactile  sensibility  over  that  just  de- 
scribed. There  was,  however,  a  continued  superiority  of  the 
snout  region  over  the  rest  of  the  surface  in  sensitiveness  to 
tactile  stimuli  and  the  use  of  the  vibrissae  as  "feelers"  became 
more  and  more  marked.  The  structural  advance  in  the  tactile 
apparatus  during  these  later  stages  is  confined  to  an  increase 
in  the  perfection  of  the  mechanism  already  described. 


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Generalizing  from  the  data  which  have  been  put  before  you 
only  in  a  greatly  abbreviated  form,  the  following  may  be  stated 
as  a  summary  of  the  results,  both  experimental  and  observa- 
tional, on  the  tactile  sense:  From  the  physiological  standpoint 
there  is  no  evidence  of  any  tactile  sense  in  the  snout  region 
before  the  formation  of  receptors  or  end-organs  in  the  form  of 
a  basket-like  reticulum  about  the  bases  of  the  anlagen  of  the 
vibrissae.  The  sense  of  touch  in  the  snout  region  becomes 
more  and  more  acute  as  the  number  of  the  terminal  sense  or- 
gans increases  upon  the  development  of  additional  vibrissae  and 
by  the  innervation  of  the  integument  generally,  into  which  the 
fibers  of  the  trigeminus  ultimately  penetrate  and  form  an  ex- 
tensive plexus. 

The  motor  reactions  to  tactile  stimuli  on  the  snout  region 
are  at  first  simple  movements  of  the  head  away  from  the  source 
of  stimulation.  As  central  connections  become  perfected  longer 
and  gradually  more  complex  reflex  arcs  are  established  and  the 
simpler  motor  reactions  of  the  earlier  stages  are  replaced  by 
such  complex  responses  as  efforts  at  the  removal  of  an  irritat- 
ing object  by  the  use  of  the  paws,  and  the  appearance  of  pain 
sensations  as  indicated  by  squeaks  of  greater  or  less  vigor,  ft 
is  probable  that  connection  with  the  cortex  has  been  formed  at 
or  even  before  birth,  though  it  has  been  impoossible  to  be  sure 
of  this  by  direct  observations. 

The  order  of  appearance  of  the  different  parts  concerned 
with  the  sense  of  touch  is  therefore  as  follows: 

1.  The  formation  of  a  primitive  reflex  arc  through  the 
simultaneous  laying  down  of  both  the  afferent  and  efferent 
nerves  and  their  connections  in  the  medulla  and  cord. 

2.  The  formation  of  effectors,  or  motor  end  organs,  upon 
the  histogenesis  of  those  muscles  that  bring  about  the  move- 
ments noted. 

3.  The  formation  of  connections  with  higher  brain  centers, 
and,  lastly,  the  establishment  of  the  function  upon  the  develop- 
ment of  the  terminal  organs  of  touch,  primitively  the  "baskets" 
in  the  follicles  of  the  vibrissae,  and  later  the  complicated  plexus 
in  the  epidermis. 

Tn  short,  the  structural  mechanism  precedes,  or  in  is  work- 
ing order  before  the  establishment  of  functional  activity,  and 
the  last  element  of  this  mechanism  to  be  formed  is  the  extero- 
ceptive end-organ. 


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9 


II.  EQUILIBRIUM: 

In  no  stage  previous  to  the  3.5  cm.  fetus  was  there  any  ex- 
perimental evidence  of  the  presence  of  a  sense  of  equilibrium. 
In  the  7l/2  mm.  embryo  there  are  no  traces  even  of  the  semi- 
circular canals;  in  the  16  mm.  stage,  the  semi-circular  canals  arc 
definitely  formed,  so  far  as  their  gross  structure  is  concerned, 
and  the  ampullae  arc  innervated  by  branches  of  the  vestibular 
nerve.  The  region  of  the  cristae  acusticae  is  indicated  merely 
by  an  elongation  of  the  endothelial  cells.  In  the  23  to  28  mm. 
embryos  the  differentiation  of  the  cells  of  the  cristae  acusticae 
is  proceeding,  but  the  sensory  and  supporting  elements  are  not 
yet  distinguishable.  A  fiber  tract  from  the  same  general  region 
of  the  medulla  in  which  the  vestibular  nerve  ends  runs  dorsad 
into  the  cerebellum  and  may  indicate  the  establishment  of  a 
connection  with  the  center  for  equilibration  in  that  part  of  the 
brain,  but  any  actual  connection  between  the  two  cannot  be  de« 
termined  in  these  preparations. 

As  already  mentioned,  the  3.5  cm.  fetus  was  the  first  stage 
observed  to  give  an  indication  of  the  possession  of  a  sense  of 
equilibrium.  One  hour  after  removal  from  the  uterus  the  young 
of  this  stage  were  able  to  maintain  an  upright  position  of  head 
and  body,  and  to  regain  this  position  when  disturbed.  When 
turned  entirely  over  onto  the  dorsum,  infrequent  and  feeble  ef- 
forts were  made  to  right  themselves.  In  the  case  of  new  born 
rats,  i.  e.,  rats  during  the  first  day  after  birth,  it  was  observed 
that  they  crawled  awkwardly  about,  turned  the  head  from  side 
to  side,  and  when  turned  over  onto  the  dorsum,  made  awkward 
righting  movements  which  sometimes  succeeded. 

The  structural  features  of  the  3.5  cm.  fetus  and  the  young 
rats  less  than  24  hours  after  birth  are  practically  identical  so 
far  as  the  apparatus  for  equilibration  is  concerned.  The  semi- 
circular canals  are  larger  than  in  the  earlier  stages  described, 
the  cristae  acusticae  have  the  sensory  and  supporting  cells 
clearly  differentiated.  The  former  are  inclosed  each  in  a 
"stockade"  of  nerve  fibers,  in  such  a  way  as  to  transmit  any 
stimulus  produced  by  a  change  in  the  position  of  the  animal. 
The  central  connections  of  the  vestibular  nerve  arc  well  defined. 

Throughout  the  later  stages  there  was  manifested  a  gradual 
perfecting  of  the  sense  of  equilibrium,  accompanied  by  a  gradu- 
ally increasing  power  of  coordination  of  movements.  These 
stages  in  fact  witness,  from  the  structural  standpoint,  the  addi- 
tion, through  the  establishment  of  various  coordination  tracts, 


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of  other  factors  concerned  in  the  final  perfection  Of  the  power 
of  equilibration,  notably  (1)  muscle  tonus,  (2)  the  use  of  the 
vibrissae,  and  (3)  sight. 

In  short,  our  experimental  and  structural  data  on  the  sense 
of  equilibrium  show  that  this  sense  is  first  apparent  upon  the 
completion  of  the  proprioceptive  end-org-an  concerned,  viz.,  the 
sensory  cells  of  the  cristae  acusticae  in  the  ampullae  of  the 
semi-circular  canals,  though  the  power  of  equilibration  is  gradu- 
ally perfected  through  correlated  development  in  other  parts, 
principally  in  the  muscular  system  (tonus),  the  eyes,  and  the 
vibrissae.  Here  as  in  the  case  of  the  sense  of  touch  the  whole 
of  the  reflex  arc  is  completed  before  the  function  is  established, 
and  the  last  link  in  the  chain  is  the  proprioceptive  end-organ. 
III.  SMELL: 

Smell  and  taste  are  two  very  difficult  senses  to  determine, 
especially  in  the  very  young  stages  with  which  for  the  most 
part  these  experiments  dealt.  In  the  case  of  embryos  from 
7l/2  mm.  to  28  mm.  long,  no  practical  means  for  testing  the 
sense  of  smell  was  devised.  However,  it  is  reasonably  certain 
that  the  sense  is  lacking,  for  the  histological  differentaition  of 
the  olfactory  epithelium  has  not  advanced  sufficiently  far  to 
enable  the  sensory  cells  proper  to  be  distinguished,  though  it 
is  during  these  stages,  especially  the  younger  ones,  that  the 
olfactory  apparatus  as  a  whole  is  being  gradually  laid  down, 
both  as  regards  its  central  and  its  peripheral  portions. 

The  3.5  cm.  fetus  showed  no  absolutely  certain  response  to 
olfactory  stimuli,  though  structurally  both  the  central  and  dis- 
tal portions  of  the  olfactory  apparatus  show  appreciable  develop- 
ment over  the  preceding  stages.  The  sensory  cells  in  the  ol- 
factory epithelium,  however,  are  not  apparently  fully  differenti- 
ated. 

During  the  first  day  after  birth  young  rats  seemed  to  per- 
ceive odors  as  evidenced  by  turning  the  head  and  movements 
of  the  snout  as  though  sniffing;  the  reaction  time  was  long,  15 
to  30  seconds.  At  this  time  structural  conditions  indicate  that 
the  olfactory  epithelium  contains  a  few  cells  at  least  which  are 
apparently  fully  differentiated  as  sensory  cells,  while  the  cen- 
tral connections  are  better  developed  than  before.  In  later 
stages  there  is,  on  the  whole,  a  gradual  perfecting  of  the  ol- 
factory sense  from  day  to  day,  accompanying  the  correlated 
differentiation  of  the  olfactory  epithelium. 

In  the  case  of  smell,  then,  as  in  those  of  touch  and  equili- 


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brium,  there  is  the  early  establishment  of  the  olfactory  nerves 
and  of  the  various  (at  least  the  chief)  olfactory  tracts  in  the 
brain  previous  to  and  independent  of  the  differentiation  of  the 
peripheral  organ  of  smell, — the  sensory  cells  of  the  olfactory 
epithelium.  In  short,  in  a  reflex  arc  involving  the  sense  of 
smell,  the  exteroceptive  organ  is  the  last  link  to  be  established, 
i.  e.,  to  become  functional. 

IV.  TASTE: 

The  3.5  cm.  fetus  could  swallow,  but  neither  in  them  nor  in 
any  preceding  stages  was  there  obtained  any  evidence  of  an 
ability  to  distinguish  between  savors  of  different  sorts.  At  no 
time  previous  to  birth  could  taste-buds  or  other  fully  differenti- 
ated organs  of  taste  be  found.  During  the  first  day  after  birth 
experimental  results  on  taste  were  very  inconclusive;  apparent- 
ly anything  of  a  liquid  nature  applied  to  the  lips  or  mouth,  es- 
pecially if  the  temperature  were  less  than  about  37.5°  C,  pro- 
duced a  feeling  of  discomfort.  Sugar-solution,  however,  was 
received  with  much  less  apparent  objection  than  salt-  or  acid- 
solutions,  but  this  may  have  been  due  merely  to  its  lack  of  the 
irritating  properties  connected  with  the  salt-  or  acid-solutions. 
No  taste-buds  are  present  at  this  time,  and  while  there  arc 
plainly  developing  in  the  fungiform  papillae  organs  which  can 
be  in  later  stages  identified  as  organs  of  taste,  they  have  not  at- 
tained their  definitive  form  and  structure  at  this  time  and  arc 
certainly  not  yet  functional. 

Though  it  was  exceedingly  difficult  to  distinguish  between 
mere  annoyance  or  discomfort  and  a  sense  of  taste,  it  was  ap- 
parent, especially  from  the  ninth  day  on  into  the  later  stages 
examined,  that  this  sense  was  present  and  gradually  being  per- 
fected. Structurally  there  was  a  concomitant  differentiation 
until  by  the  ninth  day  after  birth  the  new  type  of  taste  organ 
in  the  fungiform  papillae  had  apparently  reached  functional  ma- 
turity, while  on  and  around  the  single  circumvallate  papilla  the 
ordinary  taste-buds  are  plainly  becoming  differentiated,  though 
it  is  doubtful  whether  they  are  actually  functional  until  later 
The  two  nerves  concerned  with  the  taste  apparatus,  namely, 
the  lingual  branch  of  the  trigeminus  and  the  glossopharyngeus, 
and  their  central  connections  are  completed  long  before  birth 
and  hence  long  before  a  sense  of  taste  is  present.  In  this  case, 
then,  as  was  also  noted  for  touch,  equilibrium,  and  smell,  the  re- 
flex arc  involving  the  apparatus  for  taste  is  not  completed  until 
the  proper  exteroceptive  organs  are  fully  developed;  in  other 


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words,  here  as  elsewhere  the  organ  of  special  sense  is  t'le  last 
link  forged  in  the  chain  of  neurones  making  up  the  reflex  arc 
under  consideration. 

V.  HEARING: 

Absolutely  no  response  to  sound  was  noted  befo'c  the 
twelfth  day  after  birth.  From  that  date  until  the  sixteenth  or 
seventeenth  day  there  is  a  gradual  increase  in  the  ability  to 
perceive  sound.  Previous  to  the  twelfth  day  the  cochlear  nerve 
had  long  been  well  developed  and  its  central  connections  fully 
established,  but  while  the  exteroceptive  organ  for  sound,  the 
organ  of  Corti,  had  been  undergoing  a  gradual  development 
also,  it  had  not  reached  that  degree  of  differentiation  necessary 
for  the  perception  of  sound.  By  the  twelfth  or  thirteenth  day, 
however,  the  organ  of  Corti  is  apparently  fully  differentiated 
throughout  a  considerable  portion  of  its  extent,  namely,  about 
the  middle  third  of  its  length,  though  the  lumen  of  the  external 
auditory  meatus  is  not  fully  open.  The  next  few  days  witness 
the  completion  of  differentiation  in  the  organ  of  Corti,  and  ths 
complete  opening  of  the  meatus,  establishing  the  sense  of  hear- 
ing in  its  entirety.  Thus  the  sense  of  hearing  falls  into  line 
with  the  observations  on  the  previous  senses  considered, — 
touch,  equilibrium,  smell,  and  taste,  and  the  same  order  of 
events  occurs,  namely,  the  early  establishment  of  the  auditory 
or  cochlear  nerve  and  its  central  connections;  the  late  develop- 
ment of  the  organ  of  Corti  and  its  accessory  apparatus;  or  in 
other  words  the  appearance  of  the  sense  of  hearing  depends 
upon  the  completion  of  differentiation  in  the  exteroceptive 
organ  of  the  auditory  reflex  arc,  the  sensory  cells  of  the  organ 
of  Corti. 

VI.  SIGHT: 

Absolutely  no  response  to  light  was  obtained  before  the 
opening  of  the  eyes  on  the  sixteenth  or'  seventeenth  day.  Be- 
fore the  twelfth  day  after  birth  the  eye  is  undergoing  the  usual 
course  of  development.  By  this  time  the  rods  and  cones  are 
still  only  fairly  well  defined,  but  the  accessory  structures  are 
less  fully  developed  and  the  closed  lids  prevent  the  entrance  of 
any  but  possibly  the  very  brightest  light.  By  the  sixteenth  or 
seventeenth  day,  varying  somewhat  in  different  cases,  the  lids 
are  open  and  the  function  of  sight  is  fully  established.  The 
optic  nerve  and  its  central  connections  are  early  formed,  before 
birth  in  fact.  The  very  late  differentiation  of  the  retinal  ele- 
ments and  the  accessory  structures  of  the  eye  in  general  be- 


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13 


fore  the  power  of  sight  is  possessed  by  the  young-  rat  bring  this 
sense  into  line  with  the  conditions  noted  for  the  other  senses, 
and  reveal  the  fact  that  sight  is  not  possible  until  the  whole 
apparatus  is  in  working  order,  of  which  the  last  element  to  be 
perfected  is  the  exteroceptive  end-organ. 

The  course  of  development  of  these  special  senses  and  of 
the  structures  correlated  with  them  as  just  outlined  is  not  in 
accordance  with  theory  or  a  priori  expectation.  Following  the 
early  differentiation  of  the  neural  tube,  the  central  connections 
between  the  afferent  and  efferent  nerves  are  established  in  the 
spinal  cord,  at  least,  and  probably  also  in  the  medulla  before 
or  simultaneously  with  the  appearance  of  such  nerves,  which 
very  soon  acquire  their  proper  terminations.  The  motor 
mechanism  is  next  completed,  including  both  the  histogenesis 
of  the  muscle  tissue  and  the  formation  of  the  end-plates  of  the 
efferent  nerve-fibers;  then  there  probably  ensues  the  establish- 
ment of  the  (associational)  connections  with  the  higher  centers 
■of  the  brain;  and  last  of  all  the  peripheral  receptive  organs- 
reach  functional  capability.  This  completes  the  arc,  and  for 
the  first  time  external  stimuli  are  able  to  start  a  reaction  that 
passes  from  receptor  to  the  centers  and  thence  out  to  the  effec- 
tor. 

It  will  be  readily  perceived  that  this  is  not  the  order  of  de- 
velopment demanded  by  the  Lamarckian  theory.  If  structure 
were  to  appear  in  response  to  function,  i.  e.,  from  the  effects  of 
extrinsic  stimuli,  the  logical  order  would  be:  receptor,  afferent 
nerve,  central  connections,  efferent  nerve,  and  finally  effector. 
But  the  evidence  presented  above  shows  clearly  that  such  is 
not  the  case,  but  rather  the  reverse.  The  conclusion  therefore 
is  inevitable  that  intrinsic  forces  are  chiefly  concerned  in  the 
production  in  ontogeny  of  the  mechanism  of  these  special  senses 
and  their  effector  connections;  they  are  forces  introduced  into 
the  organism  by  heredity,  that  is,  they  inhere  in  the  organiza- 
tion of  the  germ-plasm.  The  whole  process  is  the  product  of 
germinal  organization,  though  doubtless  it  is  plastic  enough  to 
allow  for  a  considerable  degree  of  adjustment  to  minor  environ- 
mental changes. 

This  investigation  may  serve  to  emphasize  one  other  note- 
worthy fact  that  has  struck  the  attention  of  all  who  have 
studied  organic  interrelations  in  the  ontogeny  of  vertebrates, 
namely,  the  early  appearance  of  the  peripheral  nervous  system. 
Before  it  is  possible  that  distinctly  nervous  functions  can  be 


The  University  of  Oklahoma 


present,  or  if  possible,  of  any  importance  to  the  embryo,  the 
chief  nerve  trunks  are  all  laid  down,  together  with  most  or  all 
Iheir  important  branches.  For  example,  both  the  vestibular  and 
cochlear  nerves  are  well  developed  in  the  23  mm.  embryo  of 
the  rat,  if  not  earlier,  while  it  is  absolutely  impossible  that  the 
function  of  hearing  can  have  been  established.  We  have  shown 
that  in  our  experiments  the  very  first  indication  of  an  ability 
to  detect  sound  comes  not  earlier  than  the  twelfth  day  after 
birth,  yet  here  when  not  more  than  two-thirds  of  the  prenatal 
life  has  been  passed  through,  the  nerve  of  hearing  is  fully  de- 
veloped. In  the  16  mm.  embryo  the  vibrissae  have  not  yet 
pierced  the  epidermis  of  the  snout  and  it  is  hardly  conceivable 
that  the  fetus  has  need  of  a  delicate  sense  of  touch  to  maintain 
itself  within  the  amniotic  sac;  yet  the  maxillaris  and  mandibul- 
aris  branches  of  the  trigeminus  are  well  formed,  certainly  cap- 
able of  functioning,  and  terminate  in  very  large  and  complex 
exteroceptive  organs  in  the  vibrissal  follicles.  It  cannot  be 
argued  that  in  these  and  in  other  cases  not  necessary  to  be 
cited  now,  the  nerves  both  afferent  and  efferent,  and  the  recep- 
tors and  effectors  develop  in  response  to  functional  activities  or 
even  functional  needs  on  the  part  of  the  fetus  at  this  or  any 
preceding  stage  in  its  existence.  The  facts  as  stated,  however, 
demand  an  explanation,  and  so  far  as  I  am  aware  such  an  ex- 
planation has  not  heretofore  been  attempted  along  the  lines 
about  to  be  set  forth. 

Harrison,  of  Yale,  and  others  have  studied,  experimentally, 
the  earliest  stages  in  the  development  of  the  peripheral  nerves, 
and  from  their  results,  especially  from  the  cultivation  of  tis- 
sues in  vitro,  some  light,  it  seems  to  me,  is  shed  upon  this  ques- 
tion. Harrison  finds  that  all  tissues  exhibit  a  specificity  in 
their  tendency  to  undergo  each  its  own  peculiar  type  of  his- 
togenesis, into  muscle,  epithelial,  nerve,  or  connective  tissue- 
cells,  as  the  case  may  be.  This  inherent  tendency  of  embryonic 
cells  reveals  itself  irrespective  of  the  nature  of  the  external 
medium,  provided  it  be  not  detrimental  to  the  welfare  of  the 
cells  concerned.  A  neuroblast  inevitably  becomes  a  nerve-cell 
because  of  the  organization  handed  down  to  it  through  all  the 
cell-generations  from  oosperm  to  neuroblast.  This  but  con- 
firms the  conclusions  of  Whitman,  Wilson,  Conklin,  Lillic,  and 
many  others,  that  there  is  a  prelocalization  of  organ-forming 
substances  in  the  egg  at  or  before  fertilization.  In  other  words, 
the  nervous  system  develops  by  a  process  of  endogenesis  or 


University  of  Oklahoma 


15 


genesis  from  within,  i.  e.,  by  immanent  processes,  and  not  by 
epigenesis. 

Not  only  is  this  true  of  the  nervous  system  in  general,  but 
even  the  fate  of  each  neuroblast  and  its  processes  is  likewise 
predetermined.  Thus  Harrison  has  shown  in  the  case  of  the 
formation  of  the  axone  that  the  outgrowth  takes  place  "with- 
out the  application  of  any  external  physical  force  and  *  *  * 
occurs  even  when  the  normal  surroundings  are  radically  modi- 
fied. That  the  original  direction  taken  by  the  outgrowing  fiber 
is  already  determined  for  each  cell  before  the  outgrowth  actu- 
ally begins,  so  that  when  it  does  begin  it  is  dependent  upon 
forces  acting  from  within,  follows  first  from  the  fact  that  the 
nerve  fibers  within  the  embryo  tend  to  grow  out  in  a  given 
direction  even  when  quite  different  surroundings  are  substituted 
for  the  normal,  and  secondly,  from  the  fact  that  the  nerve 
fibers  which  grow  into  the  clotted  lymph,  are  there  surrounded 
on  all  sides  by  an  isotropic  medium,  which  cannot  conceivably 
be  held  to  produce  movement  in  a  definite  direction." 

Put  into  other  words,  we  may  say  that  the  various  struc- 
tures of  an  organism  are  represented  in  some  way  in  the 
oosperm,  and  make  their  appearance  not  in  direct  response  to 
the  needs  or  activities  of  the  embryo,  but  in  anticipation  of 
them,  because  of  the  inherited  forces  in  the  egg,  which  become 
localized  as  development  proceeds.  They  are  "racial  or  inher- 
ent adaptations  which  are  not  first  called  forth  by  the  conting- 
ent stimulus  to  which  they  are  the  appropriate  and  useful  re- 
sponse" (Conklin). 

It  would  appear,  then,  that  the  early  establishment  of  the 
peripheral  connections  of  nerves  receives  its  proximate  expla- 
nation in  certain  well-known  mechanical  relations  that  exist 
only  at  an  early  stage  in  embryonic  development.  Assuming 
the  truth  of  the  neurone  hypothesis,  the  question  of  how  any 
certain  nerve  reaches  unerringly  its  proper  termination  receives 
an  easy  answer,  though  no  question  in  all  neurology  has  per- 
haps been  more  warmly  debated.  According  to  Harrison's  ex- 
periments each  neurone  sends  out  its  axone  in  a  predetermined 
manner  and  direction.  This  axone  has  the  form  of  a  protoplas- 
mic process  with  a  bulb  at  its  distal  end  from  which  finger-like 
pseudopods  are  constantly  reaching  out  in  various  directions. 
The  free  length  of  this  process  may  be  as  much  as  a  millimeter 
or  a  little  more,  sufficient  at  an  early  stage  in  embryonic  de- 
velopment for  it  to  extend  from  the  center  of  origin  to  the 


16  University  of  Oklahoma 

muscle-fiber  or  epithelium  with  which  it  ultimately  connects. 
That  this  activity  must  take  place  early  in  embryonic  life  is  ex- 
plained by  the  fact  that  it  is  only  in  these  stages  that  the  neu- 
roblasts of  the  neural  tube  lie  within  the  specified  distance,  about 
a  millimeter  or  less,  from  the  parts  they  are  destined  to  in- 
nervate. That  is  to  say,  within  the  distance  through  which 
free  growth  of  a  nerve  process  is  possible.  On  the  basis  of 
adaptation  and  natural  selection,  it  is  clear  that  only  those 
embryos  which  succeed  thus  early  in  establishing  these  con- 
nections have  a  chance  to  develop  properly  and  so  to  survive. 
This  early  growth  of  the  neuroblasts  and  their  processes  is 
clearly,  from  the  standpoint  of  behavior,  a  stereotropic  re- 
sponse, as  Harrison  has  shown,  and  the  actual  connections  of 
the  fibers  with  the  particular  cells  of  the  receptors  or  effectors 
is  due  in  like  manner  to  chemotaxis,  as  Harrison  also  indicates, 
for  it  is  hardly  possible  on  any  other  grounds  to  explain  how  it 
comes  about  that  in  a  nerve-trunk  containing  both  afferent  and 
efferent  fibers,  the  latter  turn  aside  to  terminate  in  connection 
with  muscle-cells,  while  the  former  continue  in  their  course  to 
the  epithelial  sense-cells. 

Harrison  points  out  another  fact  clearly  shown  by  the  ob- 
servations here  recorded  upon  the  rat,  namely,  that  the  neuro- 
blasts, thus  early  establishing  terminal  connections  are  rela- 
tively few  in  number,  but  once  the  connection  has  been  made 
they  are  able  to  grow  in  length  whenever  and  wherever  the 
growth  and  shifting  of  organs  or  parts  go  on  around  them. 
Thus  later  nerve  processes  growing  out  from  neighboring 
neuroblasts,  in  relation  to  the  greater  functional  needs  of  the 
embryo,  or  as  other  conditons  afford  them  opportunity,  find 
even  devious  courses  already  determined  for  them  and  do  no; 
go  astray  from  the  path  to  their  own  particular  end-organs.  In 
this  way  one  can  see  clearly  the  explanation  for  the  very  devi- 
ous courses  of  some  nerves  and  can  account  for  the  complex 
distribution  of  such  a  nerve  as  the  vagus  as  it  wanders  about 
among  such  a  varied  lot  of  organs  distributed  along  so  much 
of  the  antero-posterior  axis  of  the  head  and  trunk. 
Norman,  Oklahoma,  December  11,  1915. 


UNIVERSITY  OF  ILLINOIS-URBAN  A 


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